The present invention relates to elevators and elevator systems. In particular, the present invention relates to managing an encoder failure in an elevator drive system.
Elevator systems utilizing synchronous motor elevator machines need to detect absolute angular rotor position relative to the stator pole windings to be able to achieve maximum torque. An encoder, such as an incremental encoder, may be connected to the motor to track the position of the magnet in the rotor and provide a feedback signal indicative of the position and velocity to a signal processor in the elevator system. If the feedback signal from the encoder is lost (e.g., due to a power failure), the position of the rotor is no longer known to the elevator drive system. Because this limits the control that the elevator drive system has over the motor, the elevator brake is engaged to hold the elevator car in position, and the drive is disabled. However, the time between the loss of the encoder feedback signal and detection of this condition can be substantial, resulting in uncontrolled motion of the elevator car of up to two meters.